The present invention relates to a heat transfer tube for a falling film type evaporator in which refrigerant flows onto the tube external surface to form a liquid film and evaporates so that heat is exchanged between the refrigerant and a fluid flowing through the tube.
An absorption chiller/heater is a refrigeration cycle system including an absorber in which absorbent (e.g., lithium bromide aqueous solution) absorbs vapor of a refrigerant (e.g., water), a generator that separates the vapor of a refrigerant from the absorbent, a condenser that condenses the separated refrigerant vapor, and an evaporator in which the condensed refrigerant evaporates to exchange heat. The absorption chiller/heater uses no refrigerant with a high global warming potential, such as chlorofluorocarbon.
Recently, the environmental destruction has been seriously concerned. Accordingly, the absorption chiller/heater has been developed and become widely used as an environment-friendly refrigeration cycle system for large buildings and an air conditioning system for local areas, because the absorption chiller/heater realizes a high heat efficiency without chlorofluorocarbon refrigerant that has both a high heat exchange ability and a high global warming potential. Thus, a high efficient heat transfer tube, the most important member of the absorption chiller/heater, is now strongly required.
In a falling film type evaporator such as the absorption type water cooling/heating equipment, many heat transfer tubes are provided so that their tube axes are in parallel to one another inside the evaporator body having low internal pressure. A refrigerant (e.g., water) flows onto the tube surfaces, and a heat exchange between the refrigerant and a fluid (e.g., water) flowing through the tubes is done, so that the fluid through the tubes is chilled. The refrigerant in contact with the tubes flows over the tube external surfaces. Then, the refrigerant evaporates to take away the heat of the tube external surfaces because of the low internal pressure, so that the fluid through the tubes is chilled.
A high efficient heat transfer tube needs a larger contact area between a refrigerant and a tube, namely, a larger heat exchange area. Therefore, water spreading characteristics of the refrigerant on the tube external surface need to improve.
A heat transfer tube having a plurality of projections spirally arranged on its external surface has been disclosed in JP10318691, as one example of the above described heat transfer tube for a falling film type evaporator. This heat transfer tube comprises a tube body, fins which are provided on the tube external surface and extend in a direction perpendicular to or oblique to the tube axis, and notches which extend in a direction cross to the fins and cut the fins. The depth of the notches is substantially the same as the height of the fins. The notches cut the fins to form a plurality of projections. The height of the projections is in the range of 0.2 to 0.4 mm, and the pitch between adjacent projections is in the range of 0.5 to 0.9 mm. Therefore, the water spreading characteristics of the heat transfer tube can improve compared to a smooth bore tube not having projections, and the heat transfer ability can relatively improve because the tube external surface area increases.
However, the prior art has the following disadvantages. In the heat transfer tube for a falling film type evaporator disclosed in JP10318691, the tube external surface area increases, but the water spreading characteristics on the external surface is not sufficient. Accordingly, in an evaporator in which a plurality of heat transfer tubes is arranged so that their tube axis directions are in parallel to one another, refrigerant flowing onto a top portion of the external surface of the most upper tube tends to drop in the tube circular direction before flowing in the tube axis direction. The refrigerant on the top portion of the tube external surface drops almost vertically in the tube circular direction, and then drips from a bottom portion of the tube external surface onto the next heat transfer tube. Flow of the refrigerant on the tube external surface always follows a fixed pattern, and the refrigerant always drips from an upper tube onto a fixed area of the next tube. This means that areas that the refrigerant does not reach always exist on the external surfaces of the heat transfer tubes provided inside the evaporator. As a result, a heat transfer ability of the heat transfer tubes decreases because such areas do not contribute to a heat exchange between the refrigerant and the fluid through the tubes.
The present invention aims at solving the above-described problems. The object of the present invention is to provide a heat transfer tube for a falling film type evaporator, in which water spreading characteristics of refrigerant on an external surface of the tube, especially water spreading characteristics in the tube axis direction, improve to enhance a heat transfer ability.
In one aspect, a falling film type heat transfer tube used for exchanging heat between a liquid film formed by a liquid dripping onto the tube external surface and a fluid flowing through the tube, comprises a tube body, a plurality of separate projections formed on an external surface of the tube, and a ridge which is formed in a convex shape on an internal surface of the tube and spirally extends. The projections are divided into a plurality of projection groups. In each projection group, a plurality of uniform shaped projections is spirally arranged so that the pitches between adjacent projections are uniform. The projection groups are arranged in parallel to one another. A shape of or pitch for the projections forming at least one projection group is different from the shapes of or pitches for the projections forming the other projection groups.
In the above-described structure, surface intension on each projection group is different from each other, so that refrigerant flows from one projection group to the other projection groups. The refrigerant becomes easy to flow in the tube axis direction, and the heat transfer area increases. The refrigerant drips onto various positions on a next heat transfer tube, and the heat transfer area of the next heat transfer tube also increases. Additionally, the refrigerant flows in the tube axis direction, so that the pattern when the refrigerant drips from the bottom portion of the tube is not fixed. Thus, the heat transfer area of the next heat transfer tube increases. As a result, the heat transfer area of each heat transfer tube becomes hard to dry, enhancing the ability for a heat transfer device.
In another aspect, in the falling film type heat transfer tube of the present invention, it is preferable that the shapes of all the projections are truncated quadrilateral pyramids or quadrilateral pyramids. Portions parallel to the tube axis direction increase because of the quadrilateral shaped projections. Therefore, the water spreading characteristics improve because a ratio of the amount of refrigerant flowing in the tube axis direction to the amount of refrigerant flowing in the tube circular direction increases. The thickness of a liquid film formed around pyramid shaped projections decreases compared to a columnar projection. As a result, the liquid film of refrigerant causes less prevention of heat transfer, enhancing heat transfer ability.
In another aspect, in the falling film type heat transfer tube of the present invention, three projection groups can be provided. A first projection group comprised of first projections, a second projection group comprised of second projections, and a third projection group comprised of third projections are arranged in the tube axis direction in the mentioned order.
In another aspect, the height of the first projection can be the same as that of the second projection, and higher than that of the third projection. The pitch between adjacent first projections can be the same as the pitch between adjacent second projections, and longer than the pitch between adjacent third projections. The refrigerant flows from the higher first and second projection groups to the lower third projection group, enhancing the water spreading characteristics of the refrigerant. Additionally, the refrigerant drips onto various positions on a next heat transfer tube. The refrigerant becomes easy to flow from the area of higher projections to the area of lower projections by setting the pitches between adjacent higher projections longer. As a result, the refrigerant becomes easy to flow in the tube axis direction, enhancing the water spreading characteristics.
In another aspect, the height of the first projection can be the same as that of the second projection, and lower than that of the third projection. The pitch between adjacent first projections can be the same as the pitch between adjacent second projections, and shorter than the pitch between adjacent third projections. The refrigerant flows from the higher third projection group to the lower first and second projection groups, enhancing the water spreading characteristics of the refrigerant. Additionally, the refrigerant drips onto various positions on a next heat transfer tube.
In another aspect, the height of the first projection can be higher than that of the second projection, and the height of the second projection can be higher than that of the third projection. The pitch between adjacent first projections can be longer than the pitch between adjacent second projections, and the pitches between adjacent second projections can be longer than the pitch between adjacent third projections. As a result, the refrigerant flows from the highest first projection group, via the second projection group, to the lowest third group, enhancing the water spreading characteristics of the refrigerant. Additionally, the refrigerant drips onto various positions on a next heat transfer tube.
In another aspect, in the falling film type heat transfer tube of the present invention, all pitches between adjacent projections of one projection group are preferably in the range from 0.35 to 0.95 mm.
In another aspect, in the falling film type heat transfer tube of the present invention, the heights of all the projection are preferably in the range from 0.1 to 0.5 mm.
In another aspect, in the falling film type heat transfer tube of the present invention, the pitch in the tube axis direction between adjacent projection groups is preferably in the range from 0.72 to 1.12 mm.
In another aspect, in the falling film type heat transfer tube of the present invention, the height of the ridge is preferably in the range from 0.15 to 0.45 mm.
In another aspect, in the falling film type heat transfer tube of the present invention, the angle between the extending direction of the ridge and the tube axis direction is preferably in the range from 20 to 45 degrees.
Other and further objects, features and advantages of the invention will appear more fully from the following description.